The long term objective of this proposal is twofold: a) Measure oxygen delivery from the retinal circulation to retinal tissue in the normal and diseased retina and determine whether and when this delivery becomes abnormal in the course of retinal vascular diseases such as diabetic retinopathy; b) Investigate whether oxygen diffusion through the arteriolar vessel wall and physiological shunting of oxygen plays a role in the regulation of oxygen delivery to retinal tissue. The first three years of this project will be devoted to accomplishing the following specific aims: A.1. Construction of the LDV-oximeter (LDO) and testing on whole blood flowing through tubes. This oximeter will be based on a new approach to reflection oximetry which stems from recent studies on the nature of light backscattered by red blood cells in retinal vessels. A.2. Measurements of oxygen saturation, blood flow oxygen delivery in anesthetized cats under various physiological conditions. A.3. Measurements in normal human volunteers. Establish reproducibility and sensitivity of the LDO technique. The technique proposed in this application represents a new approach to oximetry and may provide a simpler clinical tool for measuring oxygen saturation in retinal vessels than previous techniques. Furthermore, the LDO instrument may allow, for the first time, continuous measurement of oxygen delivery by the retinal circulation to the retinal tissue. It is hoped that this measurement will in the long term determine the role of oxygen delivery in the pathophysiology of vascular diseases such as diabetic retinopathy.